The combustion products (exhaust) from internal combustion engines contain poisonous nitrogen oxide compounds (NOx) and unburned hydrocarbons that are harmful to the environment. In an effort to at least partially purify the exhaust of these undesirable substances, it is long been known to use catalytic converters mounted in the exhaust flow path from the engine. While the development of catalytic converters for the small engines of automobiles is somewhat developed, federal regulations are now also requiring the makers of relatively large diesel engines to significantly reduce the emission levels of both hydrocarbons and NOx compounds.
Reducing emission levels in large diesel engines presents a new set of problems not previously encountered in relation to smaller automobile engines. For instance, the porous ceramic substrates typically utilized in catalytic converters cannot easily be extruded in diameters greater than twelve inches. This diameter substrate is simply too small to accommodate the mass flow of exhaust from large diesel engines. One manufacturer of large diesel engines has approached this problem by encasing ten or more square ceramic substrates in a metal shell, resulting in an exhaust flow cross section through a compound catalytic converter of sufficient size to accommodate the increased exhaust from the diesel engine. Unfortunately, this approach to the problem of creating a catalytic converter with a sufficient flow area suffers from the drawback of being extremely difficult to manufacture in large numbers within acceptable tolerances. What is needed is a modular catalytic converter that is easy to manufacture, reduces emissions to a satisfactory level, and is easily serviceable during the life of the diesel engine.
Because of the large effective cross sectional area required of catalytic converters for large diesel engines, the exhaust flow must necessarily diverge before encountering the catalytic converter. It has long been known that any obstacle in the exhaust flow path, including catalytic converters or muffler structure, must necessarily increase back pressure on the engine. As a general rule, increased back pressure results in lower fuel efficiency, decreased performance and a more limited altitude range for any given engine. Since the exhaust flow from the engine must also necessarily diverge significantly in that portion of the muffler devoted to attenuating low frequency noise, the present invention contemplates the incorporation of muffler structure into the same housing as the catalytic converter. Thus, a combination catalytic converter and muffler in a single housing can result in less back pressure on the engine than would otherwise occur if two separate housings were utilized.